High power Nd:YLF solid state lasers

ABSTRACT

A high power TEM 00  mode Nd:YLF solid state laser includes at least two pumped Nd:YLF solid state rods in series within a laser resonator. A spherical lens may be incorporated within the resonator as required for establishing, in concert with the rods and resonator end mirrors, a large intracavity beam diameter at the position of the rods. A cylindrical lens may be provided to compensate for astigmatic thermal focusing of the Nd:YLF rods, or else the rods may be used in pairs, in which case the rods are rotated by 90° about the laser axis with respect to each other, and a half-wave plate is inserted between the rods of each pair to maintain oscillation at a single wavelength.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No.07/421,585, filed Oct. 16, 1989, which is a continuation-in-part of U.S.patent application Ser. No. 07/342,410, filed Apr. 24, 1989.

BACKGROUND OF THE PRESENT INVENTION

a. Field of the Invention

This invention relates to solid state lasers. More specifically, theinvention is directed to an improved solid state laser including aplurality of solid state lasing elements, each having low thermalfocusing and low thermal birefringence loss, which improved laserproduces increased TEM₀₀ mode power.

b. Background of the Pertinent Art

High power TEM₀₀ solid state laser output, either continuous wave (CW)or Q-switched, is required for many commercial, scientific research anddevelopment applications. For example, diamond inscribing, laserengraving, soldering and welding applications and thick film hybridcircuit trimming operations require high power laser output. Numerousother industrial and scientific applications would also benefit ifhigher power laser output could be economically and practicallyemployed.

A large mode volume in the solid state lasing crystal, i.e., utilizingas large a fraction of the laser crystal's active volume, is a criticalrequirement in achieving high power, low order mode laser output.Unfortunately, the mode volume in neodymium (Nd) doped yttrium aluminumgarnet (YAG), commonly denoted as Nd:YAG, the most popular solid statelasing crystal, has been limited by the properties of that materialknown as thermally-induced focusing and thermal birefringence loss.

Thermally-induced focusing or thermal lensing results from the quadraticvariation in the refractive index of the Nd:YAG rod as a function ofdistance from its central axis. This variation follows, via the largepositive dn/dT, from the quadratically varying temperature dependencewhich results when a uniformly pumped (and heated) cylindrical rod iscooled on its cylindrical surface. Since the effective focal length of ahighly pumped rod can be as short as 20-30 cm (i.e., approaching the rodlength), the establishment of large mode volumes and the use of multiplerods within a single resonator is made difficult.

In addition, the radial temperature distribution results, via thethermal expansion coefficient, in a varying stress distribution. Thisstress produces, through the photoelastic effect, a birefringence in thenormally optically isotropic Nd:YAG rod. This thermal birefringencecauses a depolarization and hence an intracavity loss for a TEM₀₀ modebeam as this mode is necessarily linearly polarized. Since this lossvaries quadratically as a function of radial position from the rodcenter, the thermal birefringence also limits mode volume and therebyTEM₀₀ mode power.

Therefore, CW TEM₀₀ power over 30 watts remains unavailable from anycommercial solid state laser system today.

Certain characteristics of the crystal Nd:LiYF₄ (referred to herein asNd:YLF) have been shown in recent studies to make Nd:YLF advantageousfor use as an active lasing element. This crystal has been shown todemonstrate very low thermal focusing and, because it is a uniaxialcrystal, its natural birefringence dominates the relatively smallthermal birefringence so that thermal birefringence loss is negligible.In addition, Nd:YLF lasers have achieved TEM₀₀ power outputs comparableto Nd:YAG lasers with equivalent sized rods.

Thermal focusing in Nd:YLF laser rods, however, although small, isastigmatic, i.e., its magnitude and sign are different in the orthogonalradial directions of the crystal a and axes in a typical laser rod whoseother a axis is along the cylinder axis. This can be corrected byutilizing a cylindrical lens to produce an output beam that iscircularly symmetrical. Such a cylindrical lens is used in addition to aspherical lens normally used to expand beam diameter so as to obtainlarger mode volumes.

SUMMARY OF THE INVENTION

In accordance with the invention, a high power solid state lasercomprises a laser resonator and a plurality of solid state laser rodsmounted in pumping chambers within the resonator. The rods are arrangedin series and are constructed of a material having low thermal focusingand low thermal birefringence loss, as compared with the thermalfocusing and thermal birefringence loss of Nd:YAG. A lens means isarranged within the resonator and between the rods for establishing alarge intracavity beam diameter with respect to the laser rod diameterand, thus, a large active lasing volume in the rods. By appropriatelyselecting the radii of curvature of the resonator end reflectors, thefocal power of the lens means and various distances between systemcomponents, a Gaussian (TEM₀₀ mode) beam intensity distribution isprovided whose diameter is a substantial fraction of the laser roddiameter. That is, high laser output power is extracted from asubstantial portion of the rod volume, and thus higher TEM₀₀ laseroutput power is achieved.

It is thus seen that an object of the present invention is to increasethe TEM₀₀ power output of solid state lasers using solid state lasingcrystals which exhibit low thermal focusing and low thermalbirefringence loss.

Another object of the present invention is to increase the power outputof Nd:YLF solid state lasers.

Here, the laser rods are comprised of Nd:YLF and the laser polarizationis aligned along the crystal a axes of the rods to produce, typically,laser output at 1053 nanometers (nm). In one embodiment, a cylindricallens is employed to correct the astigmatic thermal focusing of the rods.

In another preferred embodiment, the two Nd:YLF laser rods are arrangedin series and aligned colinearly with the radial a axes of the rodsrotated 90° from one another, in order to take advantage of theoppositely signed astigmatic thermal focusing of the material on theradial a and axes. In other words, astigmatic focusing of the beam bythe first rod in the pair is corrected by the opposite astigmaticfocusing of the beam by the second rod so that a circularly symmetricaloutput beam is created. It will be appreciated that this correction orcompensation is dynamic, i.e., it operates at all laser output levels.In this embodiment, a half-wave plate is aligned colinearly andpositioned between the rods and is oriented to rotate the laserpolarization by 90° in order to maintain the appropriate laser resonancewavelength (1053 nm) in the resonator.

Thus, a further object of the present invention is to produce acircularly symmetrical output beam without the use of a cylindricallens.

For a better understanding of the preferred embodiments of the presentinvention, reference is made to the following detailed description andaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a representational view of certain components of the laser ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to increase TEM₀₀ mode volume in a solid state laser, it isnecessary to utilize as large a fraction of the cross-sectional area ofthe active laser as possible. However, in prior art Nd:YAG lasers thisincrease in beam diameter (and area) has been limited bythermally-induced focusing and birefringence loss. This limitation ofbeam expansion is overcome according to the present invention by the useof Nd:YLF rods, which have very low thermal focusing and nearly zerobirefringence loss.

In FIG. 1 there is shown a plurality of Nd:YLF laser rods placed inseries inside a resonator in order to effectively increase the overallrod length. The laser components are arranged so as to achieve a largeactive lasing volume and, thereby, a higher TEM₀₀ laser output.Generally, FIG. 1 shows first and second energy reflecting means 10, 12,which are preferably mirrors having predetermined radii of curvature.The means 10, 12 are coaxially disposed along an optical axis 15 and areseparated by a predetermined distance. Energy reflecting means 10 ispreferably a high-reflectance mirror and means 12 is preferably anoutput, i.e., partially transmissive (up to 20%) mirror. The mirrors arepreferably dielectric coatings on fused quartz substrates, as isconventional.

A lens means 20, which may be a conventional spherical lens, has apredetermined focal power and is coaxially disposed between the energyreflecting means 10, 12, with these three components defining a laserresonator. As thus described, the laser resonator may be in the form ofthe laser resonators generally designated as the "QUANTRONIX 4000Series," manufactured and sold by Quantronix Corporation of Smithtown,N.Y.

Positioned coaxially within the resonator are two substantiallyidentically sized and shaped Nd:YLF crystal rods 30, 40. The rods arecircular in cross section. In this preferred embodiment, the rods areeach 4 millimeters in diameter by 79 millimeters long. The rods 30, 40are mounted in pumping chambers 32, 42, in which light pumping sources34, 44 are also disposed for pumping the crystal rods 30, 40, as isconventional. Light pumping sources 34, 44 may comprise, for example,conventional krypton arc lamps. Laser "heads" suitable for use in thepresent invention are the QUANTRONIX Model 116 units, available fromQuantronix Corporation of Smithtown, N.Y.

Optionally but advantageously present in the laser of the preferredembodiment are a Brewster plate polarizer 50 and a Q-switch 54 which maybe either acousto-optic or electro-optic. Details of construction andthe manner of inclusion of these components into a laser resonator iswell known to those of ordinary skill in the art, as is the use of amode-lock modulator instead of a Q-switch to provide mode-locked output,if so desired.

Because the Nd:YLF rods 30, 40 exhibit some small amount of thermalfocusing, the nature of which will be discussed in more detail below,certain of the components already described are designed and positionedto provide a large intracavity beam diameter at the position of thelaser rods.

As already noted, thermal focusing in Nd:YLF laser rods is small butastigmatic, i.e., its magnitude and sign are different in the orthogonaldirections of the crystal a and axes. This difference in magnitude andsign may be handled by inserting a cylindrical lens 56 into theresonator in addition to the previously described spherical lens means20. In this case, the radial a and axes of the two rods 30, 40 arealigned. The output beam produced when such a cylindrical lens 56 isadded into the resonator is circularly symmetrical in the radial crosssectional plane of the beam. Equivalently, the desired correction couldbe achieved with the use of a compound cylindrical-spherical lens inplace of the simple spherical lens means, with the cylindrical axisaligned to compensate for the astigmatism.

A CW Nd:YLF laser according to the invention, including a cylindricallens having focal length of 130 cm to correct for astigmatic thermalfocusing, produced TEM₀₀ laser output of 40 watts, a larger power outputthan any reported value prior to the present invention.

It should be understood that there is no theoretical limit on thepresent invention to a design including only two Nd:YLF rods. Aplurality of such pumping chambers including such rods may be operatedin series, well within the spirit and scope of the invention. The onlysubstantial practical limitations are the amount of availableintracavity space and available input power.

An alternative to the use of the intracavity cylindrical lens isavailable when an even number of Nd:YLF laser rods, e.g., two, four,six, etc., are used in the resonator. Referring to FIG. 1 again, byrotating a first one 30 of each pair of two rods 90° about the axis 15relative to the second 40, astigmatic focusing of the beam by one rodcan be used to compensate for the astigmatic focusing of the beam by theother. Such rotation of one rod with respect to another, however,results in mismatch of the beam polarization with respect to the crystalaxis for appropriate laser oscillation at the 1053 nm wavelength.

In particular, it is known that a beam polarized along the crystal aaxis will oscillate at 1053 nm wavelength, while a beam polarized alongthe crystal axis will resonate at 1047 nm wavelength.

This misalignment of the crystal axes of the two rods 30, 40 is overcomeaccording to the invention by inserting a half-wave plate 58 (at the1053 nm laser wavelength) between the two rods 30, 40. The cylindricallens 56 shown in FIG. 1 is not required in such an embodiment. A beamlinearly polarized in the first rod 30 along the crystal a axis is thuseffectively rotated by 90° so as to be linearly polarized along thecrystal a axis in the second rod 40, keeping in mind that the rodsthemselves have been rotated about the axis 15 by 90° with respect toeach other in this preferred embodiment. Thus, the beam is appropriatelypolarized for 1053 nm oscillation in both rods 30, 40.

In this way, assuming uniformly dynamic astigmatic thermal focusing fromrod to rod, the total intracavity focusing is circularly symmetrical foreach pair of rods in the resonator. Thus, astigmatic thermal focusing iscompensated without the need for a cylindrical lens.

A CW Nd:YLF laser with two crystal rods, constructed according to thisembodiment of the invention, has produced a circularly symmetrical beamdelivering over 30 watts of TEM₀₀ mode output power.

A preferred embodiment of the invention using dual rotated rods fordynamic compensation of astigmatic thermal lensing is currentlymanufactured and sold by Quantronix Corporation of Smithtown, N.Y., andis designated as "Model 4216-D" which is a particular one of theQUANTRONIX 4000 Series lasers. In this laser, the energy reflectingmeans 10, 12 have convex reflective surfaces 11, 13, with the radius ofcurvature of each being 120 centimeters. Surface 11 is a high reflectorwhile surface 13 is preferably twelve percent transmissive, as iscustomary in Nd:YAG lasers as well. Lens means 20 may be a conventionalspherical lens having focal length, f, of 50 cm. The energy reflectingmeans 10, 12 are preferably spaced apart at a distance of 189.5centimeters, and the lens means 20 is positioned 97.5 centimeters fromthe high reflector 10. The cylindrical Nd:YLF laser rods 30, 40, each 4mm×79 mm, are spaced at 82 cm and 113 cm from the high reflectance means10. The half-wave plate 58 is adjacent the spherical lens means 20.Other conventional laser system components, including a mode-locker,Brewster plate polarizer, shutter and mode-selecting aperture arepreferably employed in known fashion.

With this arrangement, the Gaussian beam diameter at the rods is asubstantial fraction, i.e., sixty to seventy percent, of the laser roddiameter.

It should be understood that this overall design technique may also beemployed with more than two rods, so long as an even number of rods areused, i.e., in multiple rod/halfwave plate/rod segments.

While the invention above has been described only in terms of CW lasers,it is obvious that an intracavity Q-switch or mode lock modulator,either acousto-optic or electro-optic and activated by an appropriatedriver, can be added to produce a Q-switched or a mode-locked laseroutput.

While the foregoing description and accompanying drawing represent thepreferred embodiments of the present invention, it will be obvious tothose skilled in the art that various changes and modifications may bemade therein without departing from the true spirit and scope of thepresent invention.

We claim:
 1. A solid state laser comprising:a first energy reflectingmeans having a first predetermined radius of curvature and a secondenergy reflecting means having a second predetermined radius ofcurvature disposed facing each other along a laser axis and separated bya predetermined separation; lens means coaxially disposed at apredetermined position between the first and second energy reflectingmeans and defining, with the first and second energy reflecting means, alaser resonator, the lens means having a predetermined focal power; afirst pumping chamber disposed between the first energy reflecting meansand the lens means, including a first solid state uniaxial crystal laserrod coaxially positioned within the laser resonator, the first laser rodhaving a predefined crystal axis; and a second pumping chamber disposedbetween the second energy reflecting means and the lens means, includinga second solid state uniaxial crystal laser rod coaxially positionedwithin the laser resonator, the second laser rod having a predefinedcrystal axis; wherein the predetermined separation, and first and secondpredetermined radii of curvature, and predetermined position, andpredetermined focal power are selected so that a TEM₀₀ mode laser beamdiameter within the laser resonator at the first and second laser rodsis a substantial fraction of the diameter of the laser rods, wherein thelaser rods are aligned so that the predefined crystal axis in the firstrod is rotated by 90° about the laser axis with respect to thepredefined crystal axis of the second rod; further comprising meansbetween the laser rods for rotating only a particular beam polarization90° as the beam passes between the laser rods.
 2. The laser of claim 1,wherein the lens means is a spherical lens.
 3. The laser of claim 1wherein the first and second energy reflecting means are convex and haveradii of curvature of about 120 centimeters.
 4. The laser of claim 1,wherein said laser rods are cylindrical Nd:YLF crystal rods about 4 mmin diameter by about 79 mm in length, the lens means is a spherical lenshaving focal length of 50 cm, and the first and second energy reflectingmeans are convex and have radii of curvature of about 120 cm and arespaced apart about 189.5 centimeters.
 5. The laser of claim 1 furthercomprising:Q-switch means including Q-switch driver means, or mode lockmeans including mode lock driver means, for providing mode-locked orQ-switched laser output, coaxially disposed in the resonator along thelaser axis.
 6. The laser of claim 1 further comprising:at least a thirdpumping chamber disposed in the resonator between the first energyreflecting means and first pumping chamber, including a third solidstate laser rod made of a material having low thermal focusing and lowthermal birefringence, the third solid state laser rod coaxiallypositioned within the laser resonator; and a second lens means having apredetermined focal power, coaxially disposed at a predeterminedposition between the third and first solid state laser rods.
 7. Thelaser of claim 1 wherein the means for rotating a beam polarization is ahalf-wave plate.
 8. The laser of claim 1 further comprising:Q-switchmeans including Q-switch driver means, or mode lock means including modelock driver means, for providing mode-locked or Q-switched laser output,coaxially disposed in the resonator along the laser axis.
 9. The laserof claim 1 further comprising:at least third and fourth pumping chambersdisposed in the resonator between the first energy reflecting means andfirst pumping chamber, including third and fourth solid state uniaxialcrystal laser rods, respectively, the third and fourth solid state laserrods coaxially positioned within the laser resonator; and means betweenthe third and fourth laser rods for rotating only a particular beampolarization 90° as the beam passes between the third and fourth laserrods.
 10. The laser of claim 7 wherein the first and second uniaxialcrystal laser rods are Nd:YLF rods.
 11. The laser of claim 10 whereinthe Nd:YLF rods have a beam polarization along their a axes at awavelength of 1053 nanometers.
 12. The laser of claim 11 wherein themeans for rotating a beam polarization is a half-wave plate at awavelength of 1053 nanometers.